Method for thermally treating a flat steel product, thermally treated flat steel product and use thereof

ABSTRACT

“A method for thermally treating a flat steel product, a thermally treated flat steel product and use thereof. The method includes providing a flat steel product with a structure with a first hardness. The flat product is heated at least in sections to an austenitizing temperature. The heated flat product is cooled at least in sections so that a structure with a second hardness is formed within the flat product at least in sections, the second hardness having a higher level of hardness in comparison to the structure with the first hardness. The heating and the cooling down of the flat product are coordinated with each other such that the structure with the second hardness is formed across the thickness of the flat product and at least in one of said sections, the structure with the first hardness remains constant across the thickness of the flat product.”

The invention relates to a method for thermally treating a flat steel product. Furthermore, the invention relates to a thermally treated flat steel product and use thereof.

The use of steels with high levels of hardness and strength is known for the ballistic protection of civil as well as military vehicles, which are referred to as safety steels. Such steels generally have high levels of thickness in order to be able to completely break down the energy of an impacting object (projectile, fragment) and/or a pressure wave and to primarily prevent a penetration of the material. In order to reduce the weight and to optimize the characteristics profile, in particular across the thickness of the steels used for ballistic purposes, multilayer steels are deemed suitable, which are composed of at least two steel layers and comprise at least one combination of a hard and a second steel with a high level of ductility, wherein the hard steel destroys the impacting object and the more ductile steel should absorb the arising energy of the impact. In the prior art, different methods to manufacture multilayer steels according to this class are known which are cladded (e.g. DE 21 42 360 A), for example, by means of explosive cladding (e.g. U.S. Pat. No. 6,360,936, DE 692 02 131 T2), which is very cost-intensive, by means of hot-rolled cladding (e.g. EP 2 123 447 A1) or by means of roll cladding with the use of a heat source (e.g. DE 44 29 913 C1). All cladding methods have in common that the semi-finished products to be cladded must be processed in an elaborate, and therefore, cost-intensive manner at their connecting surfaces (e.g. see DE 43 44 879 C2, DE 10 2005 006 606 B3) in order to ensure a secure connection of the individual layers to one another so that no failure can occur between the layers in the event of a stress load, which would lead to insufficient ballistic protection. Even in the case of further processing, problems between the layers of the different steels may occur, which, for example, can become evident in the form of tension cracking, in particular when subjected to a stress load that could result due to the volatile changes in characteristics on the boundary/contact surfaces between the layers. However, diffusion processes, in particular, of interstitially solute atoms, can also prove to be problematic, such as carbon or nitrogen for example, which can penetrate into adjacent metal layers during successive thermal treatment steps, for example, in the case of hot-rolled cladding, and can influence the substance characteristics of these layers in a negative manner. The cladding methods mentioned as an example are also suitable for the manufacturing of other components, which are not designed for ballistic purposes, but, for example, for components in areas that are subject to highly abrasive factors and therefore require high wear-protection characteristics (cf. DE 10 2005 006 606 B3 as an example). Further potential for improvement exists with reference to the prior art.

The underlying object of the invention is to provide a method for thermally treating a flat steel product, which overcomes the disadvantages of the known prior art, is able to be implemented in an economic and easy manner, in particular, with comparable results, as well as to indicate a thermally treated flat product and a related use thereof.

In accordance with a first aspect of the invention, the object is thereby achieved in that a method for thermally treating a flat steel product according to the invention is indicated, which comprises the following steps:

-   -   providing a flat steel product with a structure with a first         hardness,     -   heating the flat product at least in sections to an         austenitizing temperature,     -   cooling the flat product heated at least in sections so that a         structure with a second hardness is configured within the flat         product at least in sections, which has a higher level of         hardness in comparison to the structure with the first hardness,

wherein the heating and the cooling down of the flat product are coordinated with each other in such a way that the structure with the second hardness is formed across the thickness of the flat product in sections and, at least in one section, the structure with the first hardness remains constant across the thickness of the flat product.

The inventors have found that a specific thermal treatment of a monolithic flat steel product can configure different characteristics across the thickness of the flat product, which could only be provided up until this point by means of a multilayer construction. By providing the method according to the invention, also, no problems with regard to undesired diffusion processes can occur and elaborate and cost-intensive pre-treatment and method steps can be omitted. By means of the invention, in particular, a variable hardness profile across the thickness of a monolithic steel substance can be specifically configured.

The term “structure with a first hardness” is primarily understood as meaning “the initial structure of the flat product in the delivery condition prior to the thermal treatment according to the invention”. “Hardness in the structure with the first hardness or the second hardness, etc.” is understood as meaning hardness average values across the respective sections. “Austenitizing temperature” is understood as meaning a temperature of at least A_(c1) (austenite starting temperature), particularly preferably A_(c3) (austenite end temperature). The indicated temperature values A_(c1), A_(c3) and also the martensite starting temperature M_(S) depend on the substance at hand and can be estimated based on the related alloy composition with a good level of precision.

In accordance with a first embodiment of the method according to the invention, the heating takes place at least on one side with the use of at least one heat source, wherein the structure with the second hardness is at least configured within one edge section of the flat product. The heat input into the flat product can be specifically influenced via one-sided heating. In particular, by means of this, the depth of the thermal treatment within the flat product can be controlled in sections. In order to avoid completely heating the entire thickness of the flat product or rather a complete hardening by means of cooling down after heating, the flat product can be cooled on the side facing away from the heat source, for example actively, using an appropriate means in order to prevent that at least the edge section of the flat product on the side facing away from the heat source is not primarily influenced in a negative manner. By means of this, a change in the structure can be primarily suppressed in at least one section across the thickness of the flat product and the structure with the first hardness is essentially maintained.

In accordance with an alternative embodiment of the method according to the invention, the heating takes place on both sides with the use of at least one heat source respectively, wherein the structure with the second hardness is configured in both edge sections of the flat steel product. By heating on both sides, the heat input into the flat product can be specifically influenced from both sides. In particular, by means of this, the depth of the thermal treatment within the flat product can be controlled in sections, wherein a complete heating of the flat product in its entirety, in particular the core section, should be prevented. By heating on both sides with subsequent cooling, hard edge sections can be configured with a ductile core section. It is favorable if a symmetric or asymmetric hardness profile can be configured across the thickness of the flat product depending on the depth of the thermal treatment. The hard edge sections must not have the same hardness, but can also be configured in a different manner so that, on one side of the flat product, an edge section with a structure with a second hardness is configured and an edge section with a structure with a third hardness is configured on the other side of the flat product, wherein the structure with the third hardness has a lower level of hardness than the structure with the second hardness, however, a higher level of hardness than the structure with the first hardness.

Preferably, in accordance with another embodiment of the method according to the invention, at least one inductor is used as heat source. Inductive heat sources can be operated in an economic and simple manner, and can heat workpieces at least in sections, in particular, the thermal treatment depth can be controlled in a specific and relatively simple manner. The inductor is operated, for example, with a frequency ranging between 10 Hz and 1 MHz, in particular, ranging between 100 Hz and 400 kHz. The distance between the inductor and the flat product, for example is between 1 and 10 mm, in particular, between 2 and 5 mm away from the flat product. The so-called coupling distance also determines the penetration depth in addition to the frequency. The optimal configuration with regard to operating frequency, distance and thermal treatment depth depends on the product to be manufactured and can be determined in a relatively simple matter by means of a simulation and/or trial-and-error testing.

In accordance with another embodiment of the method according to the invention, at least one edge section of the flat product is heated to at least a temperature of at least A_(c3)+20 K during the heating process and is held at this temperature for at least 1 s up to a maximum of 60 s. In the case of this temperature and holding time, it can be ensured that the area to be hardened within the flat product is also fully austenitized, meaning that the structure is fully transformed into austenite at least in the edge area. Depending on the heat source, the holding time can be reduced, for example in the case of using an inductor, to a maximum of 10 s. A temperature of more than 1,100° C. should not be exceeded in order to prevent a grain coarsening in the tempered section, whereby the characteristic could be influenced in a negative manner.

In accordance with another embodiment of the method according to the invention, the cooling takes place with the use of appropriate means, preferably the flat product is quenched with water in order to transform the previously austenitized area into a structure with a higher level of hardness, which, for example, corresponds to a primarily martensitic or martensitic/bainitic structure, wherein the quenching above a critical cooling speed depending on the substance must take place, for example, at a cooling speed of >30 K/s in order to achieve a high level of hardness.

In accordance with another embodiment of the method according to the invention, at least between an edge section with a structure with the second hardness and/or third hardness and the section with the structure with the first hardness, an annealing section with a structure with a fourth hardness is configured, which, in particular, has a lower level of hardness than the section with the structure with the first hardness. With its lower level of hardness and higher ductility in comparison to the adjacent sections, the annealing section makes an improvement possible with regard to the ballistic characteristics and further processing. The absorption capacity of an impacting object and/or forming suitability for further processing can be furthermore increased by means of providing an annealing area.

In order to have a positive effect on crack resistance, in particular, on the surface of the thermally treated area of the flat product or rather to reduce this, in accordance with another embodiment of the method according to the invention, a decarburized edge layer is configured at least in one of the edge sections with a structure with a second hardness and/or third hardness. The decarburized edge layer can, for example, be configured during heating by means of a related atmosphere, in particular, moist air and/or by means of delayed cooling after austenitizing only after the temperature goes below 700° C. As an alternative, the crack resistance at the surface can by configuring an edge section with a structure with a fifth hardness, which has a lower level of hardness in comparison to the edge section with the structure with the second hardness and/or third hardness, in particular without edge layer decarburization. This configuration takes place by means of a delayed cooling, wherein a quenching only takes place after the temperature at least in the edge layer goes below a temperature which corresponds to the M_(S) temperature.

In accordance with another embodiment of the method according to the invention, a flat steel product is used, which is ferromagnetic, which can preferably be thermally treated in an inductive manner. In particular, the flat steel product can be provided in for production reasons in the as-rolled condition, meaning that no thermal treatment has been carried out on the flat product after the last rolling pass, in particular no recrystallization annealing has been carried out. Alternatively or cumulatively, the flat product can already have a homogenous initial hardness of at least 300 HV10, for example, due to the manufacturing process. HV is the Vickers hardness and hardness testing is regulated in DIN EN ISO 6507-1:2006-03. Preferably, the flat steel product consists of the following alloy components in % by weight:

0.15 <= C <= 0.6, 0.1 <= Si <= 1.2, 0.3 <= Mn <= 1.8, 0.1 <= Cr <= 1.8, 0.05 <= Mo <= 0.6, 0.05 <= Ni <= 3.0, 0.0005 <= B <= 0.01, Al <= 0.15, Ti <= 0.04, P <= 0.04, S <= 0.03, N <= 0.03,

The remainder iron and unavoidable impurities. Preferably, the flat product is a heavy plate.

In order to generate an end product, which should not be designed to be flat, according to another embodiment of the method according to the invention, the flat product is formed and/or cut. If required, before and/or after its processing into an end product, for example, the flat product can be subjected to another thermal treatment.

According to a second aspect, the invention relates to a thermally treated flat steel product that a structure with a second hardness is formed within the flat product in sections across the thickness of the flat product and a structure with a first hardness is formed across the thickness of the flat product at least in one section, wherein the structure with the second hardness has a higher level of hardness in comparison with the structure with the first hardness and is thermally treated. Preferably, the structure with the second hardness primarily consists of a martensitic or a martensitic/bainitic structure. The section with the structure with the first hardness, however, consists of an initial structure which corresponds to the structure in the as-delivered condition and, for example, can consist of a ferritic/perlitic or tempered martensitic structure.

In order to avoid repetitions, reference will be made to favorable embodiments of the method according to the invention.

In accordance with a first embodiment of the flat product according to the invention, the structure with the second hardness is formed within an edge section of the flat product, wherein the layer thickness of the edge section can be at least 5% to a maximum of 80% of the total thickness of the flat product and the remaining thickness of the flat product consists of the section with the structure with the first hardness. By means of the asymmetrical hardness profile across the thickness of the flat product, a “harder” and “more ductile” side is made available depending on the application.

In accordance with another embodiment of the flat product according to the invention, the structure with the second hardness is formed within both edge sections of the flat product or one edge section with a structure with a second hardness is formed on one side of the flat product and one edge section with a structure with a third hardness is formed on the other side of the flat product, wherein the structure with the third hardness has a lower level of hardness than the structure with the second hardness, however a higher level of hardness than the structure with the first hardness, wherein the layer thickness of the edge section can vary between at least 5% and a maximum of 45% of the total thickness of the flat product respectively and the remaining thickness is formed by the section with the structure with the first hardness. Depending on the application, on an individual basis, an asymmetric or symmetric hardness profile can be provided within a flat product with two “hard” or “harder” sides and one “ductile” or “more ductile” core if both edge sections have the same dimensions.

In accordance with another embodiment of the flat product according to the invention, the flat product has a hardness difference of at least 100 HV10, in particular at least 150 HV10, between the at least one edge section with the structure with the second hardness and/or third hardness and the section with the structure with the first hardness. As a result, a characteristics profile optimally adapted for any application can be provided in a monolithic flat product, which has only been possible by means of a multilayer construction up until this point.

In accordance with another embodiment of the flat product according to the invention, at least between an edge section with the structure with the second hardness and/or third hardness and the section with the structure with the first hardness, the flat product comprises an annealing section with a structure with a fourth hardness, which has at least a 10 HV10 lower level of hardness, in particular at least a 20 HV10 lower level of hardness in comparison with the section with the structure with the first hardness.

In accordance with another embodiment of the flat product according to the invention, the flat product comprises a decarburized edge layer at least in one of the edge sections or comprises an edge layer with a structure with a fifth hardness, which has a lower level of hardness in comparison to the edge section. The decarburized edge layer or the edge layer with the structure with the fifth hardness can be present up to a thickness of a maximum of 10%, in particular a maximum of 5% with reference to the total thickness of the flat product.

In particular, the flat product has a total thickness between 3 and 80 mm, in particular, between 6 and 20 mm. Preferably, the flat steel product is made of a heavy plate.

In accordance with a third aspect, the invention relates to a use of the flat product according to the invention, which can be optionally formed and/or cut into an end product, as a part or component of an armoring or as a part or a component with special characteristics, in particular against of the effect of wear influences, such as abrasive and/or blast wear.

The invention is explained in detail in the following based on a drawing showing exemplary embodiments. Identical parts are referenced with same reference numbers. The figures show:

FIG. 1a ) a first exemplary embodiment for thermally treating a flat product in a schematic view,

FIG. 1b ) an illustration of the hardness progression across the thickness of the thermally treated flat product according to the first exemplary embodiment,

FIG. 2a ) a second exemplary embodiment for thermally treating a flat product in a schematic view,

FIG. 2b ) an illustration of the hardness progression across the thickness of the thermally treated flat product according to the second exemplary embodiment,

FIG. 3a ) a schematic cross section through a thermally treated flat product according to a third exemplary embodiment,

FIG. 3b ) an illustration of the hardness progression across the thickness of the thermally treated flat product in FIG. 3a ),

FIG. 4a ) a schematic cross section through a thermally treated flat product according to a fourth exemplary embodiment,

FIG. 4b ) an illustration of the hardness progression across the thickness of the thermally treated flat product in FIG. 4a ),

FIG. 5a ) a schematic cross section through a thermally treated flat product according to a fifth exemplary embodiment,

FIG. 5b ) an illustration of the hardness progression across the thickness of the thermally treated flat product in FIG. 5a ),

FIG. 6a ) a schematic cross section through a thermally treated flat product according to a sixth exemplary embodiment,

FIG. 6b ) an illustration of the hardness progression across the thickness of the thermally treated flat product in FIG. 6a ).

A first exemplary embodiment for thermally treating a flat product (1) is shown in FIG. 1a ) in a schematic view. The flat product (1) consists of a ferromagnetic steel with a primarily homogeneous structure with a first hardness (1.1), for example, of a thermally treatable steel material with a ferritic/perlitic structure with a thickness between 3 and 80 mm, preferably between 6 and 20 mm, which is preferably made of a heavy plate. The flat product (1) has a length (L), a width, which is not shown here because of the sectional view and, for example, is many times smaller than the length (L) with regard to the dimension, and has a thickness and a total thickness (D). The flat product (1) is preferably thermally treated within the scope of a continuous process at least in sections, preferably across the entire width of the flat product (1) and at least in sections, preferably across the entire length (L) of the flat product (1). As is shown in FIG. 1a ), the flat product (1) is, for example, located on a roller conveyor (R) and is moved in the direction of a thermal treatment unit (W), symbolized by the arrow shown. The thermal treatment unit (W) comprises at least one heat source for the one-sided heating of the flat product (1), wherein at least one inductor (I) is preferably used as a heat source, and at least one cooling unit to cool down the heated flat product (1), which preferably comprises at least one water shower or water spray (B). The flat product (1) is heated to at least a temperature of at least A_(c3)+20 K via the inductor (I) during the heating process and held at this temperature for at least 1 s up to a maximum of 60 s, preferably a maximum of 10 s, wherein the area to be hardened (2.1) is fully austenitized within the flat product (1). Due to the heat conduction inside the flat product (1), it must be ensured that the area (2.1) to be hardened does not exceed the desired final thickness. Depending on the thermal treatment depth, the austenitized area to be hardened (2.1) is quenched via a water spray (B), wherein the cooling speed >30 K/s is selected in order to configure a hardening structure, for example, a martensitic or a martensitic/bainitic structure (2.2) in the edge section (2). Thereby, the heating (I) and the cooling (B) of the flat product (1) are coordinated with each other in such a way that a structure with a second hardness (2.2) is formed in sections across the thickness (D) of the flat product (1′), namely in the edge section (2), and the structure with the first hardness (1.2) remains the same at least in one section (1.1) across the thickness (D) of the flat product (1′), meaning that the section (1.1) is not or is not significantly influenced by the thermal treatment in a negative manner. As an alternative and not shown here, the thermal treatment unit and/or its units can be individually arranged in a movable manner across the flat product. The hardness profile across the thickness of the flat product (1′) is shown in FIG. 1b ) and shows that the edge section (2) comprises a structure with a second hardness (2.2), which is higher than the section (1.1) with the structure with the first hardness (1.2), wherein the hardness difference is preferably at least 100 HV10.

A second exemplary embodiment for thermally treating a flat product (1) is shown in FIG. 2a ) in a schematic view. In order to avoid repetitions, only the differences in comparison with the first exemplary embodiment will be explained. The flat product (1) shown on a roller conveyor (R) is moved in the direction of a thermal treatment unit (W), as is shown in FIG. 1a ). On the side facing away from the thermal treatment unit (W), there is a second thermal treatment unit (W), which comprises at least one heat source, preferably at least one inductor (I′) to heat the flat product (1) and at least one cooling unit, preferably at least one water shower or spray (W) to cool down the heated flat product (1). The heating takes place on both sides via at least one inductor (I, I′) respectively, which preferably completely extends across the entire width of the flat product in order to cover the entire width of the flat product (1) and to completely austenitize the areas to be hardened (2.1, 2′.1) within the flat product (1). Depending on thermal treatment depth, the austenitized areas to be hardened (2.1, 2′1) are quenched via water sprays (B, B′), wherein, for example, a martensitic or a martensitic/bainitic structure (2.2, 2′.2) is configured in each case in the edge sections (2, 2′). Thereby, the heating (I, I′) and the cooling (B, B′) of the flat product (1) are coordinated with each other in such a way that a structure with a second hardness (2.2, 2′.2) is formed in sections across the thickness (D) of the flat product (1′), namely in the edge sections (2), and the structure with the first hardness (1.2) remains the same at least in one section (1.2) across the thickness (D) of the flat product (1′), meaning that the section (1.1) is not or is not significantly influenced by the thermal treatment in a negative manner and this forms the core layer (1.1) of the flat product (1′). Thereby, thermal treatment takes place simultaneously on both sides. Alternatively and not shown here, the thermal treatment units can also be arranged offset to one another, whereby both edge sections can be created in a temporally offset manner. The hardness profile across the thickness of the flat product (1′) is shown in FIG. 2b ) and shows that the edge sections (2, 2′) have a structure with a second hardness (2.2, 2′.2), which are higher than the section (1.2) or the core layer with the structure with the first hardness (1.2). Preferably, the hardness difference is at least 100 HV10.

In FIGS. 3a ), 4 a), 5 a) and 6 a), cross sections through flat products (1′) manufactured according to the invention with the related hardness profiles across the respective thickness (D) in FIGS. 3b ), 4 b), 5 b) and 6 b) are shown.

In a schematic cross section through a flat product (1′) thermally treated according to a third exemplary embodiment, a section (1.1), in particular a core layer with a structure with a first hardness (1.2), two edge sections (2, 2′) with a structure with a second hardness (2.2, 2′.2), and, respectively, an annealing section (3, 3′) with a structure with a fourth hardness (3.2, 3′.2) between the edge sections (2, 2′) and the section (1.1) are shown. The annealing sections (3, 3′) have in each case a lower hardness by at least 10 HV10 in comparison to the section (1.1). The section (1.1) and the edge sections (2, 2′) each correspond to 30% of the total thickness (D) of the flat product (1′) and the annealing sections (3, 3′) take up another 5% respectively; FIG. 3a ). The symmetrical hardness profile across the thickness (D) is shown in FIG. 3b ).

In a schematic cross section through a flat product (1′) thermally treated according to a fourth exemplary embodiment, in comparison to the third exemplary embodiment, the difference exists in the fact that the upper edge section (2) with a structure with a second hardness (2.2) is designed to be thicker, which corresponds to 50% of the total thickness (D) for example, and the lower edge section (2′) with a structure with a second hardness (2′.2) is designed to be thinner, which, for example, corresponds to 10% of the total thickness (D); FIG. 4a ). The asymmetrical hardness profile across the thickness (D) is shown in FIG. 4b ).

In a schematic cross section through a flat product (1′) thermally treated according to a fifth exemplary embodiment, a section (1.1), in particular, a core layer with a structure with a first hardness (1.2), and two edge sections (2, 2′) with a structure with a second hardness (2.2, 2′.2) are shown. In both edge sections (2, 2′), the flat product (1′) comprises a decarburized edge layer respectively or comprises an edge layer (4, 4′) with a structure with a fifth hardness (4.2, 4′.2) respectively, which has a lower level of hardness in comparison to the edge section (2, 2′). The section (1.1) is 30% and the edge sections (2, 2′) each correspond to 35% of the total thickness (D) of the flat product (1′), wherein the decarburized edge section or the edge layer (4, 4′) can be present up to a maximum thickness of 5% with reference to the total thickness (D) of the flat product (1′); FIG. 5a ). The symmetrical hardness profile across the thickness (D) is shown in FIG. 5b ).

In a schematic cross section through a flat product (1′) thermally treated according to a sixth exemplary embodiment, a section (1.1) with a structure with a first hardness (1.2), an edge section (2) with a structure with a second hardness (2.2), and an annealing section (3) with a structure with a fourth hardness (3.2) between the edge section (2) and the section (1.1) are shown. In the edge section (2), the flat product (1′) comprises a decarburized edge layer or comprises an edge layer (4.2) with a structure with a fifth hardness (4.2). The section (1.1) has a thickness of 35%, the annealing section (3) has a thickness of 5%, the edge section (2) has a thickness of 60%, from which a thickness of a maximum of 5% can be omitted for the edge layer (4.2), with reference to the total thickness (D) of the flat product (1′); FIG. 6a ). The asymmetrical hardness profile across the thickness (D) is shown in FIG. 6b ).

The design of the sections with various levels of hardness is not limited to the exemplary embodiments shown. Rather, for example, one of the edge layers can comprise a structure with a third hardness, wherein the structure with the third hardness can have a lower level of hardness than the structure with the second hardness, however, a higher level of hardness than the structure with the first hardness. In particular, due to the manufacturing process, the flat steel products can be used in the as-rolled condition as well as alternatively or cumulatively already with a homogeneous initial hardness of at least 300 HV10 for example. The flat products according to the invention, which can be optionally formed and/or cut into an end product, are either used as a part or component of an armoring or as a part or a component with special characteristics, in particular, against the effect of wear influences. Other application fields are also conceivable, in which flat products or end products with at least one section with a structure with a first hardness across the thickness and at least one section with a structure with a second hardness across the thickness of the flat or end product can be used, wherein the second hardness is greater than the first hardness.

REFERENCE LIST

1 flat product

1′ thermally treated flat product

1.1 section, core layer

1.2 structure with a first hardness

2, 2′ edge section

2.1, 2′.1 austenitized, heated area to be annealed

2.2, 2′.2 structure with a second hardness

3, 3′ annealing section

3.2, 3′.2 structure with a fourth hardness

4, 4′ decarburized edge layer, edge layer

4.2, 4′.2 structure with a fifth hardness

B, B′ cool-down spray

D thickness, total thickness

1, 1′ inductor

W, W′ thermal treatment unit 

1.-19. (canceled)
 20. A method for thermally treating a flat steel product comprising the following steps: providing a flat steel product with a structure with a first hardness, heating the flat product at least in sections to an austenitizing temperature, and cooling the flat product heated at least in sections so that a structure with a second hardness is formed within the flat product at least in sections, the second hardness having a higher level of hardness in comparison to the structure with the first hardness, wherein the heating and the cooling down of the flat product are coordinated with each other such that the structure with the second hardness is formed across the thickness of the flat product in sections and, at least in one of said sections, the structure with the first hardness remains constant across the thickness of the flat product.
 21. The method of claim 20, wherein the heating takes place at least on one side with the use of at least one heat source, wherein the structure with the second hardness is at least formed within one edge section of the flat product.
 22. The method of claim 20, wherein at least one inductor is used as a heat source.
 23. The method of claim 20, wherein at least one edge section of the flat steel product is heated to at least a temperature of at least A_(c3)+20 K during the heating process and is held at this temperature for at least 1 s up to a maximum of 60 s.
 24. The method of claim 20, wherein the cooling takes place by quenching the flat product with water.
 25. The method of claim 20, wherein the heating takes place on both sides with the use of at least one heat source respectively, wherein the structure with the second hardness is formed in both edge sections of the flat product or an edge section with a structure with the second hardness is formed on one side of the flat product and an edge section with a structure with a third hardness is formed on the other side of the flat product, wherein the structure with the third hardness has a lower level of hardness than the structure with the second hardness and a higher level of hardness than the structure with the first hardness.
 26. The method of claim 25, wherein at least between an edge section with the structure with the second hardness and/or third hardness and the section with the structure with the first hardness, an annealing section with a structure with a fourth hardness is formed, which has a lower level of hardness than the section with the structure with the first hardness.
 27. The method of claim 26, wherein at least in one of the edge sections with the structure with the second hardness and/or third hardness, a decarburized edge layer or an edge layer with a fifth hardness and a lower level of hardness in comparison to the edge section with the structure with the second hardness and/or third hardness is formed.
 28. The method of claim 20, including forming across the thickness of the flat product, a symmetric or asymmetric hardness profile.
 29. The method of claim 20, wherein the flat steel product consists of the following components in % by weight: 0.15 ≤ C ≤ 0.6, 0.1 ≤ Si ≤ 1.2, 0.3 ≤ Mn ≤ 1.8, 0.1 ≤ Cr ≤ 1.8, 0.05 ≤ Mo ≤ 0.6, 0.05 ≤ Ni ≤ 3.0, 0.0005 ≤ B ≤ 0.01, Al ≤ 0.15, Ti ≤ 0.04, P ≤ 0.04, S ≤ 0.03, N ≤ 0.03,

wherein the remainder is iron and impurities.
 30. The method of claim 20, including forming and/or cutting the flat product into a final product.
 31. A thermally treated flat steel product made according to the method of claim 20, wherein within the flat product, a structure is formed with the second hardness in sections across the thickness of the flat product and, at least in one section, a structure with the first hardness is formed across the thickness of the flat product, wherein the structure with the second hardness has a higher level of hardness in comparison to the structure with the first hardness and is thermally treated.
 32. The flat steel product of 31, wherein the structure with the second hardness is formed in an edge section of the flat product, wherein the layer thickness of the edge section is at least 5% up to a maximum of 80% of the total thickness of the flat product and the remaining thickness of the total thickness of the flat product consists of the section with the structure with the first hardness.
 33. The flat steel product of claim 31, the structure with the second hardness is formed within both edge sections of the flat product or one edge section with a structure with the second hardness is formed on one side of the flat product and one edge section with a structure with a third hardness is formed on the other side of the flat product, wherein the structure with the third hardness has a lower level of hardness than the structure with the second hardness, and a higher level of hardness than the structure with the first hardness, wherein the layer thickness of the edge section varies between at least 5% and a maximum of 45% of the total thickness of the flat product respectively and the remaining thickness is formed by the section with the structure with the first hardness.
 34. The flat steel product of claim 33, wherein the flat product has a hardness difference of at least 100 HV10 between the at least one edge section with the structure with the second hardness and/or third hardness and the section with the structure with the first hardness.
 35. The flat steel product of claim 33, wherein the flat product at least between one edge section with the structure with the second hardness and/or third hardness and the section with the structure with the first hardness comprises an annealing section with a structure with a fourth hardness, which has at least a 10 HV10 lower level of hardness in comparison with the section with the structure with the first hardness.
 36. The flat steel product of claim 35, wherein the flat product at least in one of the edge sections with the structure with the second hardness and/or third hardness comprises a decarburized edge layer or an edge layer with a structure with a fifth hardness, which has a lower level of hardness in comparison to the edge section with the structure with the second hardness and/or third hardness.
 37. The flat steel product of claim 31, wherein the flat product has a total thickness between 3 and 80 mm.
 38. The flat steel product of claim 31, wherein the flat product has a total thickness between 6 and 20 mm. 